1. Field of the Invention
The present invention relates generally to a speech signal demodulator provided in a base station in a mobile radio communications system, and more specifically to such a demodulator for demodulating speech signals applied thereto from a mobile unit using discontinuous transmission (DTX) techniques.
2. Description of Related Art
It is well known in the art to use discontinuous transmission (DTX) for reducing the power consumption of a mobile unit (or mobile station). The discontinuous transmission, which is also called VOX (voice operated transmission), allows a radio transmitter to be switched off most of the time during speech pauses for the purposes of power conservation.
The discontinuous transmission was disclosed in a paper entitled "Discontinuous Transmission (DTX) for full-rate speech traffic channels" released by ETSI/PT 12, GMS Recommendation 06.31, pages 1-13, January 1990.
Before turning to the present invention it is deemed advantageous to briefly discuss the discontinuous transmission with reference to FIG. 1.
A transmitter of a mobile station (not shown) transmits speech code sequences on a frame by frame basis while a speaker at the mobile unit is talking. As shown in FIG. 1, one frame has a time interval of 20ms (224 bits) by way of example. When the transmitter of a mobile unit detects that a speaker stops talking, the transmitter sends a post-amble to the corresponding base station. As shown, the post-amble includes two frame signals one of which is a unique word (denoted by UW1) and the other is an acoustic background noise code sequence.
Following this, the transmitter is switched off for a predetermined time duration (60 frames for example) if the speaker at the mobile unit remains silent. After the above-mentioned predetermined time duration (60 frames) elapses, the transmitter again dispatches the unique word UW1 which is followed by a new acoustic background noise code sequence. Thus, the base station receives the new background noise code sequence and updates the previously transmitted noise code sequence. The noise which is regenerated at the receive side (viz., base station) is referred to as "comfort noise". In the case where the speaker at the mobile unit continues to be silent, the combination of the unique word UW1 and a new background noise code sequence is repeatedly transmitted every 60 frames.
On the other hand, if the mobile unit detects that the speaker begins to speak again, the transmitter of the mobile unit instantly sends another unique word UW2 (viz., pre-amble) to the base station. Immediately thereafter, the mobile unit transmits speech code sequences as best illustrated in FIG. 1.
As mentioned above, if the speaker at the mobile unit stays silent for a long time, the background noise code sequences are subsequently transmitted for updating purposes. In this case, it is not seldom that a given reproduced background noise is such as to cause discomfort to the listening party at the base station. Further, it is often the case, however, that once a discomforting background noise is transmitted, this situation tends to continue for some time. Accordingly, even though the discomforting background noise issues for a mere 60 frames, it is still desirable to eliminate the same.
FIG. 2 is a block diagram showing a conventional demodulator. Speech and/or noise code sequences are transmitted, together with the unique words UW1 and UW2, from a mobile unit (not shown) to a decoder 10 which forms part of the arrangement shown in FIG. 2. A decoded code sequence is then simultaneously applied to an excitation signal generator 12, a synthesis filter coefficient generator 14, and a speech pause/start discriminator 16.
An excitation signal which is outputted from the signal generator 12, is applied to a synthesis filter 18. As is well known in the art, if the synthesis filter 18 takes the form of an all-pole type filter, then a transfer function of the filter 18 is given using a z transform. That is, ##EQU1## where N is the predetermined order of the filter, and .alpha..sub.i denotes coefficients of the synthesis filter which are applied to the filter 18.
The synthesis filter coefficient generator 14 is well known in the art and hence, the details thereof will not be described for the sake of simplicity.
The speech pause/start discriminator 16 is arranged to detect the above-mentioned unique words UW1 and UW2. If the discriminator 16 detects the uniqu word UW2, the discriminator 16 supplies a switch 20 with a control signal C1 which assumes a logic 1 level merely by way of example. The switch 20, in response to the control signal C1 assuming a logic 1 level, steers the output of the coefficient generator 14 to the synthesis filter 18. Thus, the output of the synthesis filter 18 is applied to the next stage, viz., a speech signal output circuit 22 from which a reproduced speech signal or background noise is outputted to a speaker driver (not shown) for example. The control signal C1 is also applied to a controller 24. However, the controller 24 is not responsive to the control signal C1 assuming a logic 1 level in this particular case.
On the other hand, in the case where the discriminator 16 detects the unique word UW1, the discriminator 16 outputs the control signal C1 which in turn assumes a logic 0 level. The switch 20 is responsive to this control signal C1 and applies the output of the generator 14 to the synthesis filter 18 and a memory 26.
Thus, the synthesis filter 18 reproduces the background noise which is applied to the output circuit 22 in the form of a comfort noise signal. On the other hand, the memory 26 stores the synthesis filter coefficients outputted from the generator 14. The controller 24, in response to the control signal C1 assuming a logic 0 level, instructs the memory 26 to apply the filter coefficients stored therein to the synthesis filter 18. If the speaker at the mobile unit remains silent, the above-mentioned operations continue while updating the content of the memory 26. When the discriminator 16 detects the unique word UW2, the aforesaid speech and noise signal synthesizing operations are resumed.
As above mentioned, if a given background noise induces displeasure to the listening party at the base station, he or she will quickly become annoyed.
The above-mentioned prior art has not addressed such a problem. Accordingly, it is highly desirable to eliminate this drawback inherent in the prior art.